A known driving device of a slide door for a vehicle, disclosed in JP2000-177391A and JP2000-179233, includes an electromagnetic clutch having a first rotating member, an armature member, a second rotating member and a magnetic force generating means. The first rotating member is rotated by a motor serving as a power source, the armature member is connected to the first rotating member by means of a plate spring having a flexibility in an axial direction thereof, the second rotating member is arranged so as to face the armature member in the axial direction thereof, and the magnetic force generating means generates a magnetic force by which the plate spring is deformed in such a way that the armature member is press-fitted to the second rotating member so as to rotate integrally.
Specifically, according to the driving device in JP2000-179233A, the electromagnetic clutch includes a sensor magnet fixed to the second rotating member rotating in response to a position of the slide door to be opened or closed and serving to generate a magnetic field changing in a circumferential direction thereof. The electromagnetic clutch further includes a sensor arranged so as to face the sensor magnet and serving to detect the change of the magnetic field. The sensor magnet is formed in an annular shape and is fixed to the second rotating member approximately along an outer circumferential surface by means of adhesive agent or the like.
Because the sensor magnet is fixed to the second rotating member by means of the adhesive agent, a level of mountability of the sensor magnet to the second rotating member is lowered, at the same time a manufacturing time may be extended because a time for drying the adhesive agent is needed, thereby increasing a cost of the driving device.
Furthermore, in a case where the sensor magnet is solidly fixed to the second rotating member made of other materials, because each member has a different heat expansion coefficient, the sensor magnet may be broken because of a difference of the heat expansion coefficients therebetween. Further, the sensor magnet is fixed to the second rotating member at a surface formed so as to extend in an axial direction thereof while a clearance in a radial direction is provided between the sensor magnet and the second rotating member, the sensor magnet may not be appropriately positioned in the radial direction thereof, and further, an error may occur at the sensor because of the misalignment of an axial center of the sensor magnet relative to an axial center of a rotating shaft.
According to the driving device in JP2000-177391A, the plate spring of the electromagnetic clutch is formed with a small annular portion, at which the plate spring is connected to the first rotating member by means of a tightening member such as a screw, a rivet or the like, a large annular portion at which the plate spring is connected to the armature member by means of a tightening member such as a screw, a rivet or the like, and connecting pieces, arranged in a circumferential direction of the plate spring, in order to connect the small annular portion to the large annular portion in a radial direction of the plate spring. Specifically, according to the electromagnetic clutch mentioned above, the connecting pieces of the plate spring is deformed by means of the magnetic force generated by the magnetic force generating means, so that the armature member is press-fitted to the second rotating member in order to establish an engagement therebetween so as to rotate integrally. In a state where the magnetic force generating means is not energized, the armature member is not moved toward the second rotating member so that the plate spring (at the connecting pieces) is not deformed, thereby disengaging the armature member from the second rotating member. In this state, a rotational force of the second rotating member is not transmitted to the motor, and the slide door for the vehicle is manually operatable by a user.
However, according to such electromagnetic clutch in this configuration, because the plate spring is connected to the first rotating member at the small annular portion of the plate spring by means of the tightening member such as the screw, a rivet or the like, a load is intensively applied to a connecting portion between the small annular portion and the first rotating member (e.g., the tightening member or a through hole with which the tightening member is engaged). Specifically, because the small annular portion has a small diameter arranged radially inwardly relative to the large annular portion to which the armature member is connected, the load is intensively applied to the connecting portion between the small annular portion and the first rotating member. Accordingly, the plate spring may be deformed at the connecting portion thereof, and the plate spring may be designed so as to increase its thickness more than necessary.
A need thus exist to provide an electromagnetic clutch, which is not susceptible to the drawback mentioned above.